This invention relates to an optical pick up device used in an apparatus for optically recording information on or reproducing information from a recordable optical disc.
To record information on the disc or to reproduce recorded information therefrom, an optical pick-up device irradiates, onto a disc, laser beams having appropriate intensity for recording and reproduction. At the time of recording information, a laser beam having a large light intensity, emitted from the optical pick-up device, is irradiated onto the disc to record information on the disc. On the other hand, at the time of reproducing information, a laser beam having a smaller light intensity than that for recording is emitted from the optical pick-up device and irradiated onto the disc to reproduce recorded information, on the basis of a light signal reflected by the disc surface. At the time of reproduction, a laser beam of a maximum light intensity in a range where information cannot be recorded is irradiated onto the disc, so as to keep S/N ratios of control signals of the servo system and an RF signal of the reproducing system to be sufficiently large.
Further, in the case of recordable discs reproduced by a conventional optical pick-up device, the sensitivity becomes higher when the reflectivity is lower. Since a semiconducor laser having a low output power (intensity) is conventionally used, optical discs having a small reflectivity have been used. Accordingly, since such discs are only applicable to the optical pick-up device exclusively used for discs having a small reflectivity, they cannot be reproduced by an apparatus exclusive for reproduction which can only reproduce discs having a large reflectivity.
On the other hand, there have been recently developed a recordable disc having a high reflectivity which is called an R-CD (Recordable Compact disc). At present, since information is recorded at a low linear velocity (1.2 to 1.4 m/sec) in these R-CDs, the recording can be carried out with a recording beam of relatively small power (light intensity). However, if an attempt is made to record information onto the R-CDs at a high linear velocity, in order to improve the transfer rate in future, the recording power should be higher. If the recording power is high, since the R-CDs have a large reflectivity, an excessive reflected light is incident on photodetectors (light receiving element). As a result, the focusing or tracking control servo control becomes unstable because the photodetector element itself becomes saturated.
To solve this problem, it is conceivable to provide a light quantity cut filter on the light incident side of the photodector for interrupting a reflected light having a large intensity, so as to reduce an intensity of the incident light beam by a fixed quantity with respect to the entire band at the time of recording, thus to prevent saturation of the photodetector elements. In this case, however, there was the problem that since a laser beam incident on the photodetector is reduced not only at the time of recording but also at the time of reproduction, the S/N ratio of a reproducing signal becomes small. In addition, since received light quantity of the photodetector at the time of recording differs from that at the time of reproduction, it is required to switch a gain of the servo control system between the time of recording and the time of reproduction.